The present invention relates to a pneumatically operated screw driver providing an axially driving force by a piston and rotational force by a pneumatic motor for screwing a threaded fastener into a woody member or the like.
U.S. Pat. No. 6,026,713 discloses a pneumatically operated screw driver including a driver bit engageable with a groove formed in a head of the fastener. The driver bit is connected to a piston which is driven in an axial direction of the driver bit upon application of a pneumatic pressure to one side of the piston. Further, a pneumatic motor is provided for rotating a rotary member. A rotation slide member is axially movable relative to the rotary member, and is rotatable together with the rotation of the rotary member. The piston is connected to the rotation slide member. Thus, the driver bit is axially movable while being rotated about its axis for screwing the fastener into a target. Further, a bumper is provided so as to absorb kinetic energy of the piston moving to its bottom dead center. An operation valve associated with a trigger is provided for opening a main valve in order to apply pneumatic pressure onto the piston.
The disclosed screw driver also includes a return chamber to which a compressed air is accumulatable for applying compressed air to the piston in order to move the piston and the driver bit to their initial positions. Accumulation of the compressed air into the return chamber is started when the piston is about to reach its bottom dead center. When the screw fastening operation is terminated upon abutment of the piston onto the bumper, the compressed air accumulated in the return chamber will be applied to an opposite side of the piston so as to return the piston and the driver bit to their original positions. Such conventional pneumatically operated screw driver is also disclosed in laid open Japanese Patent Application Publication No. H11-300639.
Recently, high speed screw fastening is needed, such as a screw fastening frequency the same as a nail driving frequency of a nail gun. In order to increase rotation speed of the driver bit, a pneumatic motor must provide high output. To this effect, new problems arise as to excessive frictional wear of components, particularly rotational components and heat generation of these components due to the excessive friction. To overcome the new problems, a material of the rotational components must be limited to a metal in view of heat resistivity.